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Chapter 25, Sadava
The History of Life on Earth
• Many evolutionary changes take place
over long periods of time.
• To study long-term evolutionary
change, we must think in time frames
spanning millions of years, and imagine
conditions very different from today’s.
• Fossils are preserved remains of
ancient organisms. They tell us about
body form or morphology, and where
and how the organisms lived
• Earth’s history is recorded in rocks
• Layers of rocks are called strata.
• Relative ages of rocks can be
determined by looking at strata of
undisturbed sedimentary rock (formed
by the accumulation of grains on the
bottom of bodies of water).
• The oldest layers are at the bottom,
youngest at the top.
• First observed in the 17th century by
Nicolaus Steno.
• In the eighteenth century, geologists
realized that fossils could also be used
to age rocks
• Certain fossils were always found in younger
rocks, others were found in older rocks
• Fossils in more recent strata were more
similar to modern organisms
• Radioisotopes found in the fossils can be
used to age them
• Radioisotopes can be used to determine the
actual age of rocks
• Radioisotopes decay in a predictable pattern.
• Elements can exist as different isotopes – different # of
• Half-life is the time interval over which one half of the
remaining radioisotope decays, changing into another
• Used to date igneous rocks and metamorphic rocks
• Fossils found in these rocks are determined to be the
same age as the rock
 Sedimentary rock is made up of
material from other rocks that have
been weathered, broken, and often
 Therefore, sedimentary rocks are aged by
aging the actual fossils found there
 Element for aging is Carbon
 50,000 years or younger
• In an organism, the ratio of
during its lifetime
stays constant
is an unstable isotope of carbon
• Would have vanished from Earth’s atmosphere if not for influx of
cosmic rays
• Therefore, a living organism incorporates 14C in its body from the
• When an organism dies, it is no longer incorporating
the environment
• There is no replacement of 14C and the ratio of 14C to 12C
decreases in the body
• The dead organism is buried under sediment and debris over
• This ratio can then be used to date fossils up to about
50,000 years old
• works in sedimentary rock
• Isotopes in sedimentary rock cannot be used because the
material making up the rock has come from various places and
• Dating rocks older than 50,000 years
requires estimating isotope
concentrations in igneous rocks
(formed when molten material cools)
• Decay of potassium-40 to argon-40 is used
What period are we
in now?
Dinosaurs went extinct
• The idea that land masses have moved over
time was first suggested by Alfred Wegener
in 1912
• By the 1960s, evidence of plate tectonics
convinced geologists that he was right
• Earth’s crust is divided into solid plates
about 40 km thick—collectively, the
• The plates float on a fluid layer of liquid rock or
• Heat from radioactive decay in Earth’s core causes
the magma to circulate in convection currents. This
exerts pressure on the plates and causes them to
• The movement of plates is called
continental drift
• Where plates are pushed together, they move
sideways past one another, or one is pushed
underneath the other
• Mountain ranges are pushed up, and deep rift
valleys or trenches are formed
• Where plates are pushed apart, ocean
basins form.
• Position of the continents has changed
dramatically over time.
• Position and size of land masses
influences ocean circulation patterns,
sea level, and global climate.
• Mass extinctions of marine animals
have occurred when sea level dropped,
exposing the continental shelves.
• Earth’s atmosphere has also
• The early atmosphere probably
contained little or no free oxygen
• O2 began to increase when certain
bacteria evolved the ability to use
H2O as a source of H+ ions in
photosynthesis. O2 was a waste
• Cyanobacteria formed rocklike
structures called stromatolites which
are abundant in the fossil record
• Enough O2 was liberated to allow
evolution of oxidation reactions as the
energy source to synthesize ATP
• When O2 first appeared in the atmosphere it
was poisonous to some of the anaerobic
• Some evolved the ability to metabolize the
• Advantages: Aerobic metabolism is faster and more
energy is harvested.
• Aerobes replaced anaerobes in most environments.
• Atmospheric O2 also made possible larger and more
complex cells.
• About 1.5 billion years ago, atmospheric O2 concentrations
became high enough for large eukaryotic cells to evolve.
• Further increases in O2 levels 750 to 570 million years ago
(mya) enabled evolution of multicellular organisms.
Figure 25.5 Larger Cells, Larger Organisms Need More Oxygen
• O2 concentrations increased again during
the Carboniferous and Permian periods
because of the evolution of large vascular
• Extensive burial of plant debris in swamps
formed coal deposits
• The buried organic material was not
subject to oxidation, and the living plants
were producing large quantities of O2
• O2 levels were about 50 percent higher than today’s
• It allowed evolution of giant flying insects and
amphibians that could not survive in today’s
Figure 25.6 Rising Oxygen Levels and Body Size in Insects
• Many physical conditions have
oscillated over time in response to
drifting continents, volcanic activity,
and even extraterrestrial events such
as meteorite impacts.
• Sometimes these events caused mass
extinctions in which a large proportion
of the living species disappeared.
• Earth’s climate has changed over time
• Sometimes Earth was considerably hotter
than today; sometimes colder, with extensive
• The cold periods were separated by long
periods of milder climates
• Major climatic shifts have occurred over
periods as short as 5,000 to 10,000 years,
primarily as a result of changes in Earth’s
orbit around the sun.
• Some climate changes have been even more
rapid. Extinctions caused by them appear to
be “instantaneous” in the fossil record.
• The earth is in a warming trend now.
HOWEVER, it is happening rapidly
• Today’s rapid climate change is thought to be due
to increasing CO2 concentrations, mostly from
burning fossil fuels.
• We are reversing the process of organic burial
that occurred in the Carboniferous and Permian,
but over a few hundred years rather than the
millions of years over which these deposits
• The current rate of increase of atmospheric CO2
is unprecedented in Earth’s history.
• If CO2 concentration doubles, average Earth
temperature will increase, causing droughts, sea
level rise, melting ice caps, and other major
• Collisions with large meteorites are
probably the cause of several mass
• Dinosaurs ~65mya?
• Evidence of impacts include large
craters and disfigured rocks;
compounds in the rocks with helium and
argon isotope ratios characteristic of
• The assemblage of all kinds of
organisms alive at one time (or in one
place) is called the biota.
• Flora – plants
• Fauna - animals
• Although about 300,000 species of
fossils have been described, they are
only a tiny fraction of all the species
that have existed on Earth.
• Only a tiny fraction of organisms
become fossils, and only a fraction of
those are studied by paleontologists.
• Most organisms are decomposed quickly
after death.
• If they are transported to sites with no
oxygen, where decomposition is very slow,
fossilization could occur.
• Many geologic processes transform rocks
and destroy the fossils they contain, or
bury them too deeply to be accessible.
• A large number of fossil species are
marine organisms that had hard shells or
skeletons that resist decomposition.
• Insects and spiders are also well
represented in the fossil record.